It very much depends exactly which credit card you have but unless it's a Sainsbury's Gold Card or one of a select few others then drawing cash might incur a fee. Probably the best thing you can do is call your card issuer and talk to them about it. They might send you a cheque or prefer to return the extra payment to sender.

Now a days there are so many credit card provider companies are available for making payment online specially for online shopping. But you have to choose secure as well as cheap payment gateway like merchantinc.com which is one of the secure payment gateway that does not includes application and setup fees and the standard rate for web/mail/phone transactions are 1.99%.